1. Field of the Invention
The present invention relates to an optical-fiber-amplifier measuring apparatus for measuring various characteristics of an optical fiber amplifier.
2. Description of the Related Art
FIG. 4 is a block diagram illustrating the configuration of a conventional apparatus for measuring an optical fiber amplifier in accordance with a probe method.
In FIG. 4, reference numerals 50 and 52 denote light sources. A light source whose wavelength is variable is generally used as the light source 52. Meanwhile, a light source whose wavelength is fixed is generally used as the light source 50.
The output light from the light source 50 and the output light from the light source 52 are multiplexed by a photocoupler 54, and the multiplexed light is supplied to an acoustooptic modulator 56.
In addition to the output light from the photocoupler 54, a modulating signal, e.g., a low-frequency acoustic signal, is also inputted to the acoustooptic modulator 56, but the illustration of an apparatus for outputting the acoustic signal and a detailed description thereof will be omitted herein.
The acoustooptic modulator 56 effects intensity modulation with respect to an optical signal which is inputted. The optical signal which is outputted after being subjected to predetermined intensity modulation by the acoustooptic modulator 56 is converted to two demultiplexed light beams with an optical power ratio of, for instance, 1:1 by a photocoupler 58.
One of the demultiplexed light beams is inputted to an optical fiber amplifier to be measured (hereafter referred to as the subject optical fiber amplifier) 60 so as to be amplified with a predetermined gain.
The other demultiplexed light beam branched by the photocoupler 58 is inputted to one input terminal 63a of an optical switch 62, while the optical signal outputted from the subject optical fiber amplifier 60 is inputted to the other input terminal 63b of the optical switch 62.
The optical switch 62 selects either one of the optical signal inputted from the input terminal 63a and the optical signal inputted from the input terminal 63b, and outputs the same from an output terminal 63c. Under control by an unillustrated controller, the optical switch 62 selects either one of the optical signal inputted from the input terminal 63a and the optical signal inputted from the input terminal 63b, and outputs the same from the output terminal 63c.
The optical signal outputted from the output terminal 63c of the optical switch 62 is inputted to an acoustooptic modulator 64 where the optical signal is subjected to predetermined intensity modulation and is outputted.
It should be noted that, in the same way as the above-described acoustooptic modulator 56, in addition to the optical signal outputted from the optical switch 62, a modulating signal, e.g., a low-frequency acoustic signal, is also inputted to the acoustooptic modulator 64, but the illustration of an apparatus for outputting the acoustic signal and a detailed description thereof will be omitted herein.
Reference numeral 66 denotes an optical spectrum analyzer, which is used for measuring the optical power at the aforementioned portions. Further, numeral 68 denotes a reference optical power meter for calibrating the optical spectrum analyzer 66.
Next, a description will be given of a method for measuring various characteristics of the optical fiber amplifier by the optical-fiber-amplifier measuring apparatus shown in FIG. 4.
It should be noted that a description will be given here of a method of the various characteristics of the optical fiber amplifier in a case where light is outputted from light source 52 alone, so as to simplify the description.
The method for measuring various characteristics of the optical fiber amplifier by the configuration shown in FIG. 4 is called a "pulse method".
In the pulse method, signal optical power P.sub.in inputted to the subject optical fiber amplifier 60, signal optical power P.sub.out after amplification by the subject optical fiber amplifier 60, and power (power of an amplified spontaneous emission) P.sub.ase of spontaneously emitted light (ASE light) outputted from the subject optical fiber amplifier 60 are first measured by the optical spectrum analyzer 66, respectively.
After completion of the measurement, a gain G and a noise figure NF of the subject optical fiber amplifier 60 are calculated on the basis of the signal optical power P.sub.in, the signal optical power P.sub.out, and the power P.sub.ase of the amplified spontaneous emission. The following Formulae (1) and (2) are used as formulae of this calculation: EQU G=(P.sub.out -P.sub.ase)/P.sub.in (1) EQU NF=(P.sub.ase /h.multidot..gamma..multidot.G.multidot..DELTA..gamma.)+(1/G) (2)
It should be noted that, in Formula (2) above, h represents a Planck's constant, .gamma. represents an optical frequency of the optical signal, and .DELTA..gamma. represents a measurement resolution of the optical spectrum analyzer 66.
FIGS. 5(a)-5(b) are diagrams illustrating the phase relationship between the modulated signal from the acoustooptic modulator 56 and the modulated signal from the acoustooptic modulator 64 at the time of measurement of the aforementioned signal optical power P.sub.in and P.sub.out.
When measuring the signal optical power P.sub.in and P.sub.out, the output light from the light source 52 is amplified by the subject optical fiber amplifier 60, and the power of each of the optical signals before the optical signal is inputted to the subject optical fiber amplifier 60 and after it is inputted thereto is measured. For this reason, the acoustooptic modulator 56 and the acoustooptic modulator 64 in terms of their phase relationship need to be set in the same phase.
Meanwhile, FIGS. 6(a)-6(b) are diagrams illustrating the phase relationship between the modulated signal from the acoustooptic modulator 56 and the modulated signal from the acoustooptic modulator 64 at the time of measurement of the power P.sub.ase of the amplified spontaneous emission.
The power P.sub.ase of the amplified spontaneous emission is the power of a spontaneously emitted light component (continuous light) outputted by the subject optical fiber amplifier 60.
Accordingly, to measure the power P.sub.ase of the amplified spontaneous emission, the subject optical fiber amplifier 60 needs to be set in a state in which an optical signal is not inputted to the subject optical fiber amplifier 60. For this purpose, it suffices if the phase relationship between the modulated signal from the acoustooptic modulator 56 and the modulated signal from the acoustooptic modulator 64 is set such that their phases assume opposite phases, as shown in FIGS. 6(a)-6(b), and it suffices if the power at a time when the optical signal is not being supplied to the subject optical fiber amplifier 60 is measured.
In addition, it is also necessary to effect evaluation by assuming a case where the wavelengths of the light are inputted to the subject optical fiber amplifier 60 after being multiplexed.
In this case, wavelength-division-multiplexing (WDM) signal light, in which wavelengths of light such as those shown in FIG. 7 are multiplexed, is inputted as the light source 50. Then, a light source whose wavelength is variable is used as the light source 52.
FIG. 7 is a diagram illustrating an example of the configuration of light sources for generating WDM signal light.
The example shown in FIG. 7 illustrates light sources for generating WDM signal light having four kinds of different central wavelengths.
In FIG. 7, reference 80a to 80d denote light sources for outputting optical signals with mutually different wavelengths .lambda..sub.1 -.lambda..sub.4, Reference numerals 82a to 82d denote optical attenuators for attenuating the inputted light with predetermined attenuation indices.
The optical signals outputted from the light sources 80a to 80d are respectively inputted to their corresponding optical attenuators 82a to 82d to adjust their signal light levels.
The optical attenuators 82a to 82d are connected to a photocoupler 84. Accordingly, the optical signals outputted from the optical attenuators 82a to 82d are multiplexed by the photocoupler 84, and the multiplexed light is outputted as the WDM signal light. The outputted WDM signal light is inputted to the photocoupler 54 in FIGS. 2(a)-2(c).
The purpose of using the light source 50 is to set the subject optical fiber amplifier 60 in a saturated state, and the measurement of the gain G and the noise figure NF is effected by setting the wavelength of the variable-wavelength light source 52 to a targeted wavelength. This is referred to as the measurement based on the "probe method." Incidentally, the power of the optical output from the light source 50 side is generally set to be larger than the power of the optical output from the light source 52.
In the probe method according to the above-described conventional art, outputs from the light source 52 and the light source 50 are multiplexed by the photocoupler 54, and that light is inputted to the acoustooptic modulator 56.
Since the measurement is effected by setting the phase relationship to the one shown in FIGS. 5(a)-5(b), the output light from the light source 50 for setting the subject optical fiber amplifier 60 in a saturated state and the output light from the light source 52 are modulated at the same timing.
When the signal optical power P.sub.in, the signal optical power P.sub.out after amplification by the subject optical fiber amplifier 60, and the power (power of an amplified spontaneous emission) P.sub.ase of spontaneously emitted light (ASE light) outputted from the subject optical fiber amplifier 60 are measured by the optical spectrum analyzer 66, the signal light components from the light source 50 and the light source 52 are inputted simultaneously. Therefore, in a case where the light source 50 and the light source 52 have the same wavelength components, the wavelengths are displayed in a superposed manner, so that there has been a problem in that measurement cannot be effected accurately. For this reason, it is necessary to provide a setting such that the wavelengths are not superposed on each other at the time of the measurement.